In the field of auto racing, there are several methods in which to predict the performance of a race car. One method uses horsepower correction factors. However, this method does not take into consideration different engine sizes, engine combinations, carburetion, or body styles a race car can have, thereby failing as a performance indicator. Since every car is unique and will react in its own way to changes in the atmosphere, one must develop a method to relate the atmosphere to the car's performance.
The key variable in determining the effect of the atmosphere on the car's performance is the amount of oxygen present in the atmosphere. However, the oxygen content is not directly measured. Rather, the density altitude, which indicates the density of the gas elements in the atmosphere, is measured as an index of the car's performance. Oxygen, as one of the gas elements in the atmosphere, is estimated based on the average breakdown of the various gas elements in the atmosphere. Oxygen's percent of concentration is assumed constant in relation to these other gases. Therefore, as the density of the gas changes in the atmosphere, the density of the oxygen also changes in relative proportion.
In order to characterize fully the effect of the atmosphere on the automobile's performance, multiple calculations of different density altitude values are required. One must then plot the natural curve of the car's performance based on the density altitude versus the engine performance. Thus, using statistical regression analysis, a driver can predict the performance of the car based on the plot of the density altitude values.
However, for the density altitude plot to be an effective indicator of the car's performance, it must be particularly accurate. Small changes in density altitude or inaccurate measurements can be the difference between winning and losing in auto racing. As little as a 100 foot change in density altitude can noticeably change the performance of an automobile. In fact, most racers do not realize that a minor change in the atmosphere can equate to a major change in density altitude. On average, a 1.degree. change in temperature, a 0.1 inHg change in pressure, or a 10% change in humidity can cause more than a 100 foot change in density altitude.
Instruments being used in other weather stations have variables that make it impossible to measure density altitude at a level needed to predict the performance of a race car. These inaccuracies can stem from a person's inability to correctly distinguish what the instrument is reading or to correctly calculate the density altitude.
Further, inaccuracies may come from the effect temperature has on the instrument. For example, a change in temperature can have an effect on a barometer or a hydrometer. A barometer is factory calibrated at 70 degrees. Then it is exposed to the heat of 90 degrees, but with the same pressure. The components that make up this barometer can expand or contract causing an error in the reading. Temperature compensated barometers are available, but they are costly. The effects on a hydrometer are similar.
Finally, inaccuracies can come from the instruments lack of accuracy or repeatability. For example, a sling psychrometer's accuracy depends on how fast and how long one slings it, the pressure to which it is exposed, and the contamination of the wick. The reading from a sling psychrometer will differ when taken on two days of the same humidity, but different pressures. Thus, when using standard weather instruments (digital or mechanical), one would be lucky to compute density altitude to plus or minus a few hundred feet. This amount of error in density altitude calculation is too great to be a reliable indicator of automobile performance and is therefore not acceptable.